Bug 370185: clean up view header includes (trivial). rs=bzbarsky.

widget/src/cocoa/nsChildView.mm

@@ -55,6 +55,7 @@
 #include "nsIRollupListener.h"
 #include "nsIEventSink.h"
 #include "nsIScrollableView.h"
+#include "nsIViewManager.h"
 #include "nsIInterfaceRequestor.h"
 #include "nsIServiceManager.h"
 #include "nsGfxCIID.h"